This invention relates generally to radio frequency antenna feed structures and, more particularly, to feed structures having septum polarizers.
As is known in the art, in many radio frequency communication systems, a pair of independent signals are transmitted and received as a composite signal of circularly polarized energy. More particularly, each one of a pair of signals is transmitted and received with a corresponding one of two senses of polarization of the composite circularly polarized signal; i.e., one of the pair of signals as a right-hand circularly polarized energy component and the other one of the pair of signals as a left-hand circularly polarized energy component. Such systems therefore require the use of an antenna feed having a pair of electrically isolated feed ports. During transmission, each of the feed ports is fed by a corresponding one of a pair of radio frequency signals. It should be noted that the feed ports may be fed simultaneously or at different periods of time. The feed then combines the two signals into composite circularly polarized energy; the right-hand sense polarized component of such energy carrying one of the pair of signals and the left-hand sense polarized component of such energy carrying the other one of the pair of signals. During reception the feed operates in a reciprocal manner. That is, the composite circularly polarized energy received by the feed is separated by the feed into a right-hand circularly polarized energy component which carries one of a pair of signals and a left-hand circularly polarized component which carries the other one of the pair of signals. The feed then couples the right-hand circularly polarized component to one of the pair of electrically isolated feed ports and couples the left-hand circularly polarized component to the other one of the pair of feed ports.
As is also known in the art, one desirable type of feed is a coaxial feed 10. Here, the feed includes an outer conductor and an inner conductor. The circularly polarized energy travels along the length of the feed between the inner and outer conductors. One such feed is shown in FIGS. 1, 2 and 3. Such feed 10 includes two separate devices: (A) a rear orthogonal mode transducer (OMT) 12; and (B) a forward waveguide quarter-wave polarizer 14 having a pair of dielectric vanes 16. The OMT 12 includes a pair of feed ports 18, 20 electrically isolated by conductive plates 22 which extend between the inner conductor 24 and the outer conductor 26 along a diameter of the coaxial feed 10, as shown more clearly in FIG. 2. The waveguide quarter-wave polarizer includes the dielectric vanes 16, such vanes extending along a diameter of the feed 10, such diameter being at a 45 degree angle with respect to the conductive plates 22 (i.e., a septum) to thereby convert between circularly polarized energy and linearly polarized. Thus, for example, on receive, right-hand circular energy is converted into horizontal (linear) polarization and the left-hand circularly polarized energy is converted into vertically polarized energy. The horizontal polarized energy passes to one of the pair of electrically isolated ports and the vertically polarized energy passes to the other one of the electrically isolated ports. Reciprocally, linearly polarized energy introduced into one of the electrically isolated feed ports is converted into circularly polarized energy with one sense of polarization, for example, right-hand circularly polarized energy. While such a feed operates satisfactorily in many applications, it is a relatively large structure and requires lossy dielectric materials. Further, because the dominant mode in a coaxial waveguide is the TEM mode, and in the application described above the desired modes are the TE11 vertical and TE11 horizontal modes, any successful coaxial septum polarizer design must provide these desired modes while carefully avoiding excessive excitation of the TEM mode.
In accordance with one feature of the invention, a waveguide feed structure is provided having a coaxial transmission line. A conductive, planar septum is disposed in, and along a diameter of, the transmission line. A feed port is electrically coupled to the transmission line. The septum has a rear portion disposed proximate the feed port. The feed port and the rear portion of the septum are arranged to establish an electric field in the transmission line between the inner conductor and the outer conductor with a component substantially perpendicular to the planar conductive septum. A forward portion of the septum is asymmetrically disposed along the diameter to establish an electric field component within the transmission line along said diameter of the transmission line.
In one embodiment, a pair of feed ports is provided. The rear portion of the septum is disposed proximate the feed ports to electrically isolate one of the feed ports from the other one of the feed ports.
In one embodiment, a waveguide feed structure is provided having a coaxial transmission line. A conductive, planar septum is disposed in, and along a diameter of, the transmission line. A feed port is electrically coupled to the transmission line. The septum has a rear portion disposed proximate the feed port, such rear portion of the septum extending between the inner conductor and the outer conductor. The feed port and the rear portion of the septum are arranged to establish an electric field in the transmission line between the inner conductor and the outer conductor with a component substantially TE11 mode along a direction perpendicular to the planar septum. A forward portion of the septum is asymmetrically disposed along the diameter to provide a gap between the inner conductor and the outer conductor, such gap establishing an electric field component within the transmission line having a TE11 component along said diameter of the transmission line. In one embodiment, the septum has a pair of distal ends. One of the ends is separated from a proximate portion of the outer conductor with a distance of such separation being different from a distance between the other one of the pair of ends and a proximate portion of the outer conductor. In one embodiment, the first-mentioned distance increases along the transmission line from the rear portion of the septum to the forward portion of the septum.
In one embodiment, the distance is increased in steps to provide a phase shift to energy propagating along the transmission line between a distal end of the septum and the outer conductor. In one embodiment the phase shift is approximately 90 degrees over the frequency band of operation.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.